1. Field of the Invention
The present invention relates to a manufacturing method of a silicon film having crystallinity or a silicon compound film (for instance, a SiGe semiconductor film) having crystallinity. For example, the invention can be applied to manufacture of a thin-film transistor.
2. Description of the Related Art
The thin-film transistor (hereinafter abbreviated as xe2x80x9cTFTxe2x80x9d) is known which uses a thin-film semiconductor. This is formed by using a thin-film semiconductor, particularly a silicon semiconductor film, that is formed on a substrate. While the TFT is used in various kinds of integrated circuits, it now attracts much attention particularly as a switching element provided for each pixel of an active matrix liquid crystal display device and a driver element formed in peripheral circuit sections of the same. The TFT now attracts much attention also as a device indispensable for multilayered integrated circuits (three-dimensional ICs).
As for a silicon film used in the TFT, it is simple and convenient to use an amorphous silicon film formed. However, the electrical characteristics of a resulting TFT are far lower than those of a TFT using a single crystal semiconductor for semiconductor integrated circuits. Therefore, at present, this type of TFT can be used for only limited purposes such as a switching element of an active matrix circuit. A silicon thin film having crystallinity may be used to improve the characteristics of the TFT.
Silicon films having crystallinity other than a single crystal silicon film are called a polysilicon film, a microcrystal silicon film, etc. Such a silicon film having crystallinity can be obtained by forming an amorphous silicon film and crystallizing it by heating (thermal annealing). This method is called a solid-phase growth method because conversion from an amorphous state to a crystal state is effected while the solid phase is maintained.
However, the silicon solid-phase growth has problems that the heating temperature and time need to be set at more than 600xc2x0 C. and more than 10 hours, respectively, and that it is difficult to use an inexpensive glass substrate. For example, the Corning 7059 glass, which is commonly used in the active matrix liquid crystal display device, has a glass strain point of 593xc2x0 C. Therefore, in view of increasing the substrate area, it is problematic to subject this glass to thermal annealing of more than 600xc2x0 C. for a long time.
In connection with the above problems, studies of the present inventors have revealed that crystallization can be completed by performing heating at 550xc2x0 C. for about 4 hours by depositing, by a very small amount, a certain kind of metal element such as nickel or palladium on the surface of an amorphous silicon film and then heating it. Naturally a silicon film even higher in crystallinity can be obtained by annealing of 600xc2x0 C. and 4 hours.(refer to Japanese Unexamined Patent Publication No. Hei. 6-244103 (JP-A-6-244103).
To introduce a very small amount of metal element (for accelerating crystallization), various methods are available such as depositing a coating of a metal element or its compound by sputtering (refer to JP-A-6-244104), forming a coating of a metal element or its compound by such a means as spin coating (JP-A-7-130652), and forming a coating by decomposing a gas containing a metal element by thermal decomposition, plasma decomposition (JP-A-7-335548), or the like. Selection may be made properly among those methods in accordance with their features.
It is also possible to introduce a metal element selectively, i.e., into a particular portion, and then cause crystal growth to proceed from the portion where the metal element is introduced to the periphery by heating (called xe2x80x9clateral growth methodxe2x80x9d). Having directivity in crystallization, a crystalline silicon film produced by this method exhibits much superior characteristics when used properly in connection with the directivity.
It is also effective to further improve the crystallinity by performing illumination with strong light such as laser light after the crystallization step using a metal element (JP-A-7-307286). It is also effective to perform thermal oxidation after execution of the above-mentioned lateral growth method (JP-A-7-66425).
By performing crystallization by using a metal element in the above-described manner, a crystalline silicon film having better quality can be obtained at a lower temperature in a shorter time. The temperature of the heat treatment, which strongly depends on the kind of amorphous silicon film, is preferably set at 450xc2x0-650xc2x0 C., even preferably at 550xc2x0-600xc2x0 C.
However, the most serious problem of the above method is necessity of removing the introduced metal element. Adverse effects on the electrical characteristics and reliability of a metal element that is introduced in a silicon film are not negligible. In particular, because of the mechanism of the crystallization using a metal element, the metal element remains in the coating mainly as a conductive silicide and hence becomes a major cause of defects.
It is known that in general metal elements (particularly nickel, palladium, platinum, copper, silver, and gold) can be captured by crystal defects, phosphorus, etc. For example, JP-A-8-330602 discloses a technique for reducing the concentration of metal elements in a channel forming region by implanting phosphorus ions into a silicon film with a gate electrode as a mask and then performing thermal annealing (furnace annealing) or optical annealing (laser annealing or the like), thereby allowing metal elements that are contained in the silicon film to move to source and drain regions and fixing (gettering) the metal elements there.
In the technique of JP-A-8-330602, when phosphorus is implanted into the source and the drain, the silicon film is rendered amorphous and the number of crystal defects increases, whereby metal elements are gettered by phosphorus and crystal defects. The region where to implant phosphorus is not limited to the source and the drain, and may be any region except at least a region where a channel forming region is to be formed. It is apparent to a person having ordinary skill in the art that metal elements can be removed though the degree of removal depends on the distance from the phosphorus-implanted region.
To enable the gettering, annealing needs to be performed for a sufficient time for metal elements to move to the phosphorus-implanted region. Therefore, thermal annealing is suitable for this purpose. However, in general the annealing temperature effective for the gettering is more than 600xc2x0 C. (it depends on the kind of metal element). Executing a process of such a high temperature for a long time increases a possibility that the substrate is deformed and hence becomes a factor of causing mask misalignment in later photolithography steps.
On balance, optical annealing is preferable. JP-A-8-330602 does not discuss a light source of the optical annealing and the embodiments include a statement that an excimer laser is used. However, the pulse width of excimer lasers is shorter than 100 nsec and experiments have shown that light illumination in such a short time cannot provide a sufficient gettering effect.
The present invention has been made in view of the above-described problems and an object of the invention is therefore to present conditions suitable for optical annealing and thereby provide a method effective in removing a catalyst element.
The basic concept of the invention is to heat a region from which a metal element is to be removed at a sufficiently high temperature for a sufficient time by optical annealing. The known rapid thermal annealing (RTA) method is preferable for the heating for a sufficient time.
Where the RTA method is used, high gettering efficiency can be obtained by heating of 1 second to 10 minutes though it depends on the temperature. Further, this method can heat only a particular material without the need for directly heating a substrate.
Still further, this heating process provides an effect of improvement in crystallinity in addition to the gettering action.
A crystalline silicon film obtained by utilizing a metal element for accelerating crystallization of silicon is in a polycrystalline state. The RTA decreases the number of dangling bonds at grain boundaries and inactivates grain boundaries, which is favorable for improving the device characteristics of devices manufactured.
However, there is the following problem. A phosphorus-implanted region is highly light-absorptive and hence is sufficiently heated by optical annealing (laser annealing), because it is amorphous and phosphorus exists there. In contrast, a region concerned from which a catalyst element is to be removed is highly transparent and hence is not heated sufficiently, because it is crystalline.
Therefore, the temperature of the phosphorus-implanted region becomes higher than that of the region from which the metal element is to be removed. As a result, the amount of the metal element moving from the former region to the latter region is not negligible as compared with the amount of the metal element moving from the latter region to the former region, resulting in reduction in gettering efficiency.
Naturally a large part of the metal element atoms existing in the phosphorus-implanted region are fixed because of the existence of a large amount of phosphorus and defects capable of capturing those, but a certain part are rendered movable in the above-mentioned state. In conclusion, a sufficient effect cannot be obtained unless the temperature of the region from which the metal element is to be removed is approximately equal to that of the phosphorus-implanted region.
According to the invention, in a step in which RTA is performed, the entire area of a silicon film is covered with an amorphous silicon film which is highly light-absorptive, whereby applied energy is effectively absorbed and hence the entire area of the silicon film is given a sufficiently high temperature. That is, a region where to effect gettering and a phosphorus-implanted region are heated uniformly. With this measure, nickel is allowed to move to the phosphorus-doped region with high efficiency; that is, nickel can be gettered with high efficiency.
In particular, it is even more preferable to have the amorphous silicon film contain a group 15/Va element such as phosphorus because the light-absorptiveness is increased. Further, as disclosed in JP-A-8-213316, the amorphous silicon film itself also exhibits a gettering effect because it has gettering ability due to defects included therein.
To implement the above concept, the invention provides a manufacturing process comprising the steps of: selectively forming a mask on a crystalline silicon film that has been obtained by utilizing a metal element for accelerating crystallization of silicon; accelerating and implanting a group-15 element into a region of the crystalline silicon film that is not covered with the mask; forming an amorphous silicon film so as to cover the mask and the crystalline silicon film; and heating the crystalline silicon film to a high temperature by illuminating it with strong light, to thereby allow the metal element to move from a region of the crystalline silicon film that is covered with the mask to another region of the crystalline silicon film (RTA step).
According to another aspect of the invention, there is provided a manufacturing process comprising the steps of: forming a mask on a crystalline silicon film that has been obtained by utilizing a metal element for accelerating crystallization of silicon; forming an amorphous silicon film so as to cover the mask and the crystalline silicon film; accelerating and implanting a group 15/Va element; and heating the crystalline silicon film to a high temperature by illuminating it with strong light, to thereby allow the metal element to move from a region of the crystalline silicon film that is covered with the mask to another region of the crystalline silicon film (RTA step).
One important point of the invention is the thickness of the amorphous silicon film. It is preferable that the thickness of the amorphous silicon film be more than 1,000 xc3x85. If the amorphous silicon film is too thin, the light absorptiveness is insufficient. Where light is applied from above the substrate in the RTA step, the heat conduction becomes insufficient if the amorphous silicon film is too thick. In this case, it is preferable that the thickness of the amorphous silicon film be less than 5,000 xc3x85.
Similarly, the heat conduction becomes insufficient if the mask that is provided between the amorphous silicon film and the crystalline silicon film is too thick. It is preferable that the thickness of the mask be less than 2,000 xc3x85. A thin (10-100 xc3x85) film such as a silicon oxide film may be provided between the mask and the crystalline silicon film to improve the adhesion between those.
It is preferable that the material of the mask be different from the material of the undercoat of the crystalline silicon film. This is because the undercoat is also etched in etching the mask if the mask and the undercoat are made of the same material since the phosphorus-implanted portion of the silicon film does not exist after completion of the gettering.
In the RTA step, light may be applied from below the substrate, i.e., from the back surface side.
In the present invention, the portion of the silicon film from which the metal element is to be removed is heated to 600xc2x0-1,200xc2x0 C., preferably 700xc2x0-1,000xc2x0 C. In the RTA method, since a portion that absorbs light is heated in a concentrated manner, the temperature of the substrate is kept much lower than the above temperature range and hence the influences of the RTA on the substrate are negligible.
The metal element may be one or a plurality of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au. In particular, the use of nickel (Ni) is most desirable in terms of reproducibility and effect.
In addition to phosphorus (P), other such as N, As, Sb, and Bi may be used. However, phosphorus can attain the highest effect. In particular, the use of phosphorus is most desirable when nickel is used as the metal element for accelerating the crystallization. This is because compounds of nickel and phosphorus exist in various states as exemplified by Ni3P, Ni5P2, Ni2P, Ni3P2, Ni2P3, NiP2, and NiP3 in very stable manners. That is, nickel and phosphorus tend to combine with each other easily and the combined states are very stable.
During the gettering, grain boundaries in a silicon film obstruct movement of the metal element. In general, in a silicon film immediately after solid-phase growth, the metal element precipitates in grain boundaries in the form of silicides to cause the grain boundaries to grow. Since such silicides are stable thermodynamically (the metal element precipitates in drain boundaries because precipitated states are more stable thermodynamically), the metal element is not prone to escape from the grain boundaries. Grain boundaries cause another problem that they capture and fix metal element atoms coming from other portions.
In contrast, where laser annealing is performed by illuminating pulse laser light to a silicon film that has been crystallized by solid-phase growth, the tendency that the metal element precipitates in grain boundaries is greatly reduced. This is because the duration of the pulsed laser annealing is too short to allow stabilization in terms of thermodynamics (particularly in a case where the pulse width is 1 xcexcsec or less) and hence the growth of grain boundaries is insufficient. For this reason, in a silicon film after pulsed laser annealing, a large part of metal element atoms exists in a dispersed manner. Since such metal element atoms are highly mobile and grain boundaries large enough to capture those atoms is small in number, the gettering can be effected efficiently.
In the present invention, it is preferable that the concentration of the group 15/Va element be one order or more higher than that of the metal element, and be as high as 5xc3x971019 to 2xc3x971021 atoms/cm3. At the same time as the group-15 element is implanted, hydrogen, oxygen, nitrogen, or carbon may be implanted at 1xc3x971019 to 1xc3x971021 atoms/cm3. These elements, when introduced by a large amount, obstruct the crystallization by the RTA, and hence can maintain the number of defects in a portion in which the group-15 element has been implanted. Further, the transparency is increased in a silicon film having a high concentration of carbon, nitrogen, or oxygen. Therefore, the phosphorus-implanted portion is made less light-absorptive and hence the heating is suppressed there.
The group-15 element may be added in advance, i.e., at the time of forming an amorphous silicon film. Also in this case, it is preferable that the concentration of the group-15 element be set within the above-mentioned range.
The present invention is also characterized in that the gettering is performed before or during a step of defining the active layer of a transistor by etching a silicon film.